The present invention relates to a medium controller for medium-exchange type memory units that is used as peripheral equipment of a computer (for example, an autoloader in a library of magnetic tape units or optical disk units).
The events, such as opening or closing of the entry/exit door, resetting or stopping power supply for the medium controller, are hereafter referred to as medium entry/exit-enable actions. When power supply is stopped or a reset button is pressed, it becomes possible to open or close the door of, the medium storage and, after that, a medium may be taken out or put into storage. This is the reason why those events are treated as the medium entry/exit-enable actions. Upon recognizing any of such events, the medium controller has medium storage information with the contents before this event automatically updated or the system site operator issues a command to have the information updated.
Conventionally, when medium storage information is to be updated automatically or by the operator issuing a command, the prior-art medium controller is unable to obtain medium entry/exit information because there is no sensor to recognize a medium in the medium storage area. As a solution, generally, the medium transfer unit (accessor) is equipped with a sensor or a bar-code reader to read a bar code on the medium and the medium transfer unit is made to access all the medium storage places (hereafter referred to as slots) where media are stored, and each time the sensor or the bar-code reader, provided for (accompanying) the accessor, recognizes a medium, the medium storage information is updated. In the prior-art medium transfer unit, each time some medium is replaced manually, the medium transfer unit accesses all medium storage places as mentioned above, taking a long time. Therefore, there is a long waiting time before the medium controller can be accessed from the system.
The conventional medium controller cannot grasp the medium storage status unless the medium transfer unit (accessor) sequentially checks all the slots; therefore, there are the following two problems.
The first problem is that, in a medium controller with a very large number of media in storage, the accessor checks every slot to see what medium is stored. This requires a long time for updating medium storage information. For this reason, in many cases, medium storage information is updated only when the operator desires. Consequently, when a medium is taken out or put into storage especially by manual operation, if the operator does not update medium storage information, it follows that the medium storage information does not match the actual storage status.
The second problem is that, in a medium controller with fewer media in storage, medium storage information is updated automatically in many cases when the condition that media are exchanged occurs, with the result that each time a medium comes in or out, the medium controller is kept waiting while the medium transfer unit is accessing all the slots and cannot perform other processing.